a) Field of the Invention
The present invention relates to a process for producing an urethane compound from a formamide compound or an amine compound and dimethyl carbonate as starting materials. An urethane compound is readily convertible to a corresponding isocyanate compound by thermal decomposition and thus can be used as an intermediate material for producing the isocyanate. Particularly an aliphatic diisocyanate compound derived from an aliphatic urethane compound having two carbamate ester groups in the molecule is useful as a raw material for producing polyurethane or polyurea having a distinguished resistance to yellowing.
b) Related Prior Art
An urethane compound can be produced from a nitro compound, an amine compound or a formamide compound as a main starting material. For example, a process using a nitro compound as a main starting material includes, for example, a process for carbonylation by reduction based on reaction of a nitro compound with carbon monoxide in the presence of an alcohol, as disclosed in JP-A-55-147253. A process using an amine compound as a main starting material includes a process for carbonylation by oxidation based on reaction of an amine compound with carbon monoxide and molecular oxygen or a nitro compound in the presence of an alcohol, as disclosed in U.S. Pat. No. 4,297,501. In these processes for carbonylation by reduction or that by oxidation, an expensive catalyst of platinum group metal such as Pd, Rh, etc. must be used, and reaction must be carried out at a high temperature under high pressure, resulting in an increase in the catalyst cost and capital investment. Thus, the processes suffer from these industrial disadvantages.
U.S. Pat. No. 4,398,036 discloses a process based on reaction of an amine compound with a carbamate ester or an amine compound with urea and alcohol. However, the process suffers from such disadvantages as a low reaction rate, that is, a low space-time yield, much formation of high boiling point by-products from the carbamate ester or urea, and the resulting difficulty in separation and recovery of the urethane compound.
Various processes based on reaction of an amine compound with dimethyl carbonate in the presence of a catalyst have been proposed as a process using an amine compound as a main starting material (U.S. Pat. No. 3,763,217; EP No. 48,371; U.S. Pat. No. 4,395,565). As a catalyst, a Lewis acid catalyst, a lead, titanium or zirconium-based catalyst, an alkali catalyst, etc. are used. However, these catalysts generally have a low reaction rate, and N-alkyl compounds are readily formed as by-products, resulting in a decrease in the yield of urethane compound. The processes suffer from these disadvantages. JP-A-64-85956 discloses a process based on a specific amine compound that hardly causes N-alkylation reaction as a side reaction, where the water content of dimethyl carbonate must be made to less than 0.2% and the amine compound and an alcoholate catalyst must be continuously or intermittently added to the reaction system.
The present inventors proposed a process based on reaction of a formamide compound with dimethyl carbonate in the presence of an alcoholate catalyst or reaction of an amine compound with dimethyl carbonate in the presence of an alcoholate catalyst and methyl formate (EP No. 0436800A1). The urethane compound obtained in the presence of an alcoholate catalyst of EP No. 0323514A1 and EP No. 0436800A1 can be converted to corresponding isocyanate by thermal decomposition.
However, in these processes, side reactions are considerably accelerated by the alkali catalyst in the urethane thermal decomposition step and thus it is indispensable to completely remove the catalyst. According to the conventional procedure, the alkali catalyst is neutralized by adding an acid to the reaction product solution, and the precipitated salt is separated from the solution. However, when the acid is excessively used or the neutralized salt has a high solubility, the salt itself gives an adverse effect on the thermal decomposition. Thus, extraction or washing with water is indispensable, and complicated purification procedures such as distillation, etc. are also indispensable. Use of the alkali catalyst has these disadvantages.
The present inventor also proposed an improvement of the process of EP No. 0436800A1, that is, a process based on reaction of a formamide compound with dimethyl carbonate, or reaction of an amine compound with dimethyl carbonate in the presence of methyl formate, at 100.degree. to 200.degree. C. without using any catalyst (JP-A-4-235954). The proposed process is distinguished in reaction in absence of catalyst, but it is inevitable to elevate the reaction temperature, and thus the resulting methyl formate is partly decomposed into methanol and carbon monoxide, and a high pressure reactor is required. Thus, the proposed process still has problems to be solved.
As described above, many processes have been as far proposed for the production of an urethane compound, but there have been still problems to be solved with respect to the urethane yield, separation and recovery of catalysts, treatment of by-products, purification of an urethane compound, higher capital investment, etc., and thus development of an industrially distinguished process for the production of an urethane compound has been keenly desired.